"We need to evaporate and sweat to cool down but when temperatures get close to or above our core temperature, and when humidity is high, the air becomes saturated and we're not going to lose that sweat so our cooling mechanism is hampered," said Dr Hanna.

The conditions wouldn't affect everyone equally but in a major heatwave in India, she said, people could start to overheat even when sitting still.

Under a high emission scenario, India could experience heatwaves that cause death within hours by the latter part of this century. Picture: MIT.

'RIVER OF FIRE'

"If it's 38C outside people feel crappy and grumpy and that has an impact on assaults so it has all manner of social issues," she said.

In August, the Territory Government kicked off a project to see where Darwin's hots spots were - and what was causing them - so they could cool the CBD down.

The heat mitigation study uses a dedicated "energy bus" and drones to measure surface and air temperatures.

"The study found our streets, parking lots, roofs and pavements have very high surface temperatures, ranging from 45-67C," said Chief Minister Michael Gunner at the time.

"Areas such as the Post Office carpark, the Supreme Court car park, and the Bus Terminal are incredibly hot - Cavanagh Street (the CBD's main thoroughfare) is a river of fire."

Prof Samtamouris told news.com.au Darwin was a classic case of an urban heat island where materials used in roads and buildings turbo charged temperatures.

A Darwin heat mitigation study has found some surface temperatures are in excess of 60C. Picture: UNSW

BITUMEN

However, temperatures drop dramatically in areas of foliage. Picture: UNSW.

"Black surfaces like bitumen absorb high amounts of solar radiation leading to high surface temperatures." he said.

"A material with a temperature of about 70C may heat the air by around 3C."

Alternative materials, such as special "cool" asphalt, can bring the surrounding temperatures down.

"In Darwin, you have overheating because there's too much bitumen and not enough greenery".

The study will continue for the next year but the government said it is already burying one of its major car parks to reduce its impact on air temperatures.

And it's not just Darwin. Sydney's west is regularly up to 10C warmer than the CBD. The reasons are different - the CBD is cooled by winds coming off the seas which peter out by the time you reach, say, Penrith.

But the result is the same - increasingly uninhabitable cities. And climate change, pushing average temperatures up, continues to stymie mitigation efforts.

"Townsville and Cairns are not as bad but they will start to become like Darwin. Everything is just moving to the extreme but we just don't know exactly when or how fast it will happen." said Prof Hanna.

"Global temperatures are going so badly and emissions are increasing so much that it's not looking good."

Planting more trees and creating shady streets was a good strategy to make cities more liveable, she said. But a few plants here and there had their limits.